Electronic communications devices communicate with each other in accordance with radio communications protocols. One such radio communications protocol is the Bluetooth protocol. The Bluetooth protocol specifies different power modes in which an electronic communications device can operate. Minimum requirements for the operation of the device in each power mode are specified by the protocol. One of these requirements is that the operations of the device have to be clocked with a prescribed accuracy. In the case of the Bluetooth Specification v4.0, the active clock has to be accurate to 250 ppm (parts per million) in the low power “sleep” mode of operation.
Typically, one or more crystal oscillators are used to run the operation of a Bluetooth chip during an active mode, for example when the chip is transmitting or receiving Bluetooth signals. However, crystal oscillators are high frequency and have a high power consumption. Low power analogue oscillators are preferred for use in lower power modes of operation. They are lower frequency and have a much lower power consumption than crystal oscillators. However, low power oscillators are only thermally stable in a small window around room temperature. Outside this window, they exhibit poor temperature performance. Thus, they cannot support the required accuracy specified by the Bluetooth Specification v4.0 across the range of temperatures to which the devices they are incorporated into are routinely exposed.
One approach to satisfying the required clock accuracy in the low power sleep mode is to actively run both a crystal oscillator and a low power analogue oscillator. The low power analogue oscillator runs the operation of the chip, but is correlated against the crystal when needed. This ensures that the required clock accuracy can be guaranteed even across those temperature ranges where the low power oscillator in isolation exhibits poor thermal stability.
This approach solves the problem of providing the required clock accuracy. But the crystal oscillator has a high power consumption, and thus this solution increases the power consumption of the low power sleep mode. With increased market demand for lower power/longer battery life electronic devices, a lower power solution is needed.